A UN human rights official on Tuesday urged two US states to halt the execution of two mentally ill prisoners on death row a day before they were due to die.

The two men -- Warren Hill and Yokamon Laneal Hearn -- were convicted of murder in separate cases in Georgia and Texas.

Their death sentences were upheld despite claims that each has mental disabilities and as such their executions are unconstitutional, the Office of the High Commissioner for Human Rights (OHCHR) said in a statement.

"It is a violation of death penalty safeguards to impose capital punishment on individuals suffering from psychosocial disabilities," said special rapporteur Christof Heyns.

Heyns said there was a risk that "other governments would follow the same approach in justifying the imposition of the death penalty for people suffering from psychosocial disabilities rather than applying a more humane punitive measure".

Hill, who has spent 21 years on death row, was sentenced to death in 1991 for murdering a fellow prisoner. He was originally in jail for murdering his girlfriend, according to state authorities.

He was sentenced to death despite a US Supreme Court decision in 2002 banning the execution of convicts with intellectual disabilities.

The ruling nonetheless left each US state with the authority to determine what constitutes mental disability. Georgia maintains mental retardation must be proved "beyond a reasonable doubt".

"This higher standard of proof, making it very difficult to demonstrate that one actually suffers from a psychosocial disability may, I fear, mean that Mr Hill, scheduled for execution tomorrow, would be a fatality in violation of international as well as domestic law," said Heyns.

The UN expert added that there was evidence to suggest that Hearn -- on death row since 1998 and due to be executed in Texas -- "also suffers from psychosocial disabilities" linked to alcohol abuse by his pregnant mother.

Sapa-AFP

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